diff --git a/include/boost/optional/detail/optional_aligned_storage.hpp b/include/boost/optional/detail/optional_aligned_storage.hpp
index 2937349..b290921 100644
--- a/include/boost/optional/detail/optional_aligned_storage.hpp
+++ b/include/boost/optional/detail/optional_aligned_storage.hpp
@@ -28,7 +28,7 @@ class aligned_storage
     // BOOST_MAY_ALIAS works around GCC warnings about breaking strict aliasing rules when casting storage address to T*
     union BOOST_MAY_ALIAS dummy_u
     {
-        char data[ sizeof(T) ];
+        unsigned char data[ sizeof(T) ];
         BOOST_DEDUCED_TYPENAME type_with_alignment<
           ::boost::alignment_of<T>::value >::type aligner_;
     } dummy_ ;